Allow a user to specify a custom output column name for a metric

[stuartlynn]

It would be great if a user could specify a custom name for an output metric instead of just getting the one in the parquet file.

They could specify

• One of the existing identifiers from the metadata : hxl, human readable name, id etc
• A fully custom string that they specify

This could be part of the MetricSpec enum

So in the recipe file format something like

{"MetricId": {"hxl": "...", "rename_to":"outputname"}}

Or something similar. We would need to think about how to handle regex / fuzzy id's in that situation.